The recent unprecedented increases in the prices of oil and natural gas has prompted new efforts in developing commercially feasible coal gasification processes, in order to effectively utilize available coal reserves. A severe obstacle encountered in bringing coal gasification to a widely utilized commercial reality is the disposal, in an environmentally acceptable manner, of the sour water produced by the gasification process. This air and water pollution problem associated with the coal gasification process will, in the future, take on added significance as stricter environmental legislation becomes effective.
Coke oven gases, gases from coal gasification and gases from shale oil processing contain contaminating amounts of ammonia, hydrogen cyanide, carbonyl sulfide, carbon dioxide, and hydrogen sulfide. In order to remove these contaminants, the gases are often scrubbed with water, and then steam-stripped to separate the dissolved gases from the water which is subsequently reused or discarded. It is advantageous, for both environmental and commercial reasons, to produce elemental sulfur through use of the Claus process. However, the high ammonia content of the gas in question may cause severe plugging of the catalyst beds employed in the Claus process. Therefore, the ammonia content of the gas stream must be significantly reduced prior to the stream being fed to the Claus plant.
A traditional method of disposing of the water-washed ammonia-containing gas stream has been simply to discharge the stream into the municipal sewer system. This water frequently contains appreciable amounts of hydrogen sulfide, which is a serious pollutant and a toxic gas. In light of recent legislation and public concern, the discharge of ammonia and sulfur rich aqueous solutions into municipal sewer systems has become an unacceptable, if not illegal, method of disposing of the gasification process waste effluent.
A second method involves the steam stripping of the sour water and the subsequent incineration of the stripped gases to convert the hydrogen sulfide in the stripped gas to sulfur dioxide for release to the atmosphere. Processes of this type are usually not economically feasible when performed in conjunction with coal gasification, since they require the heating of the stripped gas to very high temperatures, e.g. on the order of approximately 2200.degree. F. Therefore, this method is usually only suitable in coke-oven plants where a high temperature heat source is readily available (see U.S. Pat. Nos. 3,540,189, 3,822,337 and 3,661,507).
Methods have also been proposed for scrubbing the stripped gases in order to produce a saleable product. One such method involves the scrubbing of the stripped gases with sulfuric acid for the production of ammonia sulfate. Another process, the so-called "Phosam" process, effects ammonia removal by scrubbing the stripped gases with an aqueous ammonium phosphate solution and produces anhydrous ammonia. Both of these processes suffer from high capital cost requirements and the inability to generate sufficient revenues to recover operating costs.
The prior art also contains numerous proposals for disposing of ammonia streams that are not the result of coal gasification. For instance, U.S. Pat. No. 3,467,491 discloses an apparatus for combusting pure ammonia streams by use of air over a platinum catalyst to produce water and nitrogen. U.S. Pat. No. 3,804,596 teaches the combustion of a pure ammonia vapor stream in a refrigeration unit. Neither apparatus is suitable for use in a coal gasification process, because, inter alia, of the contaminants contained in the ammonia-rich stripped gas stream. Also, the use of an oxidation catalyst, such as copper oxide, has been used for ammonia removal from the effluent of a combustion potassium-carbonate acid removal process (see, e.g., U.S. Pat. No. 3,412,736). Again, apart from any other considerations, this process is not suitable when sulfur compounds are present in an ammonia stream, since the oxidation catalyst would be poisoned by the sulfur.